1. Field of the Invention
The present invention relates to an X-ray imaging apparatus configured to generate an X-ray image of a subject using an X-ray beam passing through the subject.
2. Description of the Related Art
Conventionally, a computed tomography (CT) apparatus for generating a three-dimensional cross-sectional image of a subject is used in the medical area. In order to obtain the three-dimensional cross-sectional image, radiation emitted to the subject from a circumference of the subject is detected and converted into an image signal after it passes through the subject. Then, through reconstruction processing, the image signal is converted into a three-dimensional cross-sectional image.
A helical scanning CT apparatus is a type of apparatus which is commonly used today. The helical scanning CT apparatus is configured such that a radiation source and a radiation detector, configured as a pair, rotate helically around the subject with a virtual body axis of the subject as their axis of rotation to detect intensity of transmitted radiation at a plurality of positions.
On the other hand, in recent years, a CT apparatus has been developed in which a subject, which is positioned between a fixed radiation source and a fixed radiation detector, is rotated in generating a tomographic image. This type of CT apparatus includes a two-dimensional plane sensor as the radiation detector. By emitting a conical radiation beam to the subject, this CT apparatus can generate fluoroscopic images which are necessary in reconstructing a three-dimensional image by a single scan. Further, this CT apparatus eliminates the need for a continuous movement of the subject in the direction of the body axis, which is essential to the common CT apparatus.
An apparatus by which the subject is rotated while its image is taken is discussed in Japanese Patent Application Laid-Open No. 2005-205082. According to this apparatus, it is important to confirm safety of the subject and then rotate the subject after its position is stabilized. A CT apparatus which satisfies such needs is being developed nowadays.
Japanese Patent Application Laid-Open No. 2005-205082 discusses an image taking method in which the subject is rotated while its image is taken. Further, an apparatus discussed in Japanese Patent Application Laid-Open No. 2005-205082 has the capability to secure the safety of the subject and stabilize the posture of the subject during the rotation, which is required during the image taking process.
For example, Japanese Patent Application Laid-Open No. 2006-43193 discusses an apparatus in which a supporting member used for controlling a body motion of the subject is set on a rotating platform. The subject, taking a half-crouching position, is supported by this supporting member on the rotating platform. Further, the CT apparatus discussed in Japanese Patent Application Laid-Open No. 2005-205082 provides a lower limb guard in front of the subject. With this guard, a lower limb of the subject is prevented from contacting a non-rotating portion around the subject during the rotation. By rotating the subject instead of the radiation source and the detector, CT images can be taken with enhanced throughput. Accordingly, CT images are replacing general still images in the area of chest screening.
Further, in order to secure the subject safely, the CT apparatus discussed in Japanese Patent Application Laid-Open No. 2006-43193 detects a state of a member supporting the subject and controls a rotation of a rotation table based on the result of the detection.
However, since radiation detectors of the aforementioned imaging apparatuses are large-sized two-dimensional plane sensors, these imaging apparatuses are generally expensive. Further, from the viewpoint of effective use of a limited installation space in a hospital, a demand for an X-ray imaging apparatus which is capable of taking two different types of images, that is, the CT image and the general still image, is increasing. Japanese Patent Application Laid-Open No. 2005-205082 discusses such an imaging apparatus. When a still image is taken using this imaging apparatus, a space is made on the rotating platform by moving the subject supporting member away from a range of X-ray emission.
In taking a CT image with such an imaging apparatus, the subject is preferably positioned in a close vicinity of an X-ray imaging unit so long as the subject does not contact the unit during the rotation. This contributes to providing a wider reconstruction area, and further, to reducing a possibility of a blurred image. Accordingly, in the actual image taking, it is necessary to set the X-ray imaging unit at a position nearer to the subject compared to the position of the X-ray imaging unit discussed in Japanese Patent Application Laid-Open No. 2005-205082.
In the actual image taking, a lower limb Pa of a subject P seated on a rotating table 2 passes adjacent an X-ray imaging unit 3 while the subject is exposed to the X-ray beam irradiated from an X-ray tube 1 as shown in FIG. 5. Further, if a lower limb guard 4 is provided in front of the subject P, the lower limb guard 4 will rotate under the X-ray imaging unit 3 as well as the lower limb Pa. Thus, in taking a CT image, the X-ray imaging unit 3 needs to be positioned within an imaging range L1.
On the other hand, in taking a still image, as discussed in Japanese Patent Application Laid-Open No. 2005-205082, the subject supporting member is moved away from the imaging area by a moving unit located under the subject supporting member, and the subject is moved closer to the X-ray imaging unit. Then the image is taken while the subject is in full contact with the outer portion of the X-ray imaging unit. On the other hand, the CT apparatus is required to be capable of taking a general still image of a wider range, e.g. the whole body of the subject, from the head to the lower limb. This is because still images are generally taken when patients go through a first medical testing. Therefore, the X-ray imaging unit 3 is required to move up and down in a wider range while a still image is taken compared to when a CT image is taken.
However, if the X-ray imaging unit 3 is configured to cover a wide movable range for the still images, then, the X-ray imaging unit 3 tends to move in a more than necessary range for the CT images. For example, if the X-ray imaging unit is set at a position inappropriately low for the CT imaging, the subject supporting member or the lower limb Pa of the subject P may contact the X-ray imaging unit 3 during rotation. In order to ensure safety, attention needs to be paid when setting the position of the X-ray imaging unit 3. As a result, ease of operability of the X-ray imaging apparatus is reduced.
A similar problem occurs when the image taking mode is changed. If a general still image is taken at a low position by the X-ray imaging unit 3, and then the mode is changed to the CT mode and the subject supporting member is moved, the lower limb Pa of the subject P may contact the X-ray imaging unit 3 if the rotating table 2 is rotated without changing the position of the subject. In this case, safety is reduced as described above.
Furthermore, with respect to a subject-rotating CT imaging apparatus including a plane sensor, a movement of the subject in the body axis direction, which is necessary in the common helical scanning CT apparatus, is not required. On the contrary, images need to be taken from many different angles from a certain height during the CT imaging. Thus, if an imaging apparatus including a height-adjustable X-ray imaging unit moves the X-ray imaging unit up and down during the CT imaging, proper images may not be obtained and therefore reconstruction becomes difficult. Since the conventional apparatus does not include a method for prohibiting the up-and-down movement during the rotation, there is a possibility that the X-ray imaging unit moves up-and-down during the image taking.